Quartz 4

3. Read-Write Lock Attributes

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1. The Goal: Sharing Between Processes

By default, a read-write lock allocated in memory is valid only for threads within the same process.

  • The Problem: If you fork a process or map a region of shared memory, you might want a Reader-Writer lock that resides in that shared memory to coordinate the parent and child (or unrelated processes).
  • The Solution: You must explicitly set the Process Shared attribute.
2. The Setup Sequence

Just like with mutexes, you cannot simply assign a value to change this behavior. You must use an attribute object. The Functions:

  1. pthread_rwlockattr_init: Initializes the attribute object.
  2. pthread_rwlockattr_setpshared: Sets the scope to PTHREAD_PROCESS_SHARED.
  3. pthread_rwlock_init: Initializes the actual lock using the attribute.
  4. pthread_rwlockattr_destroy: Cleans up the attribute object.
3. Code Example

Here is how you would initialize a read-write lock that resides in shared memory so that multiple processes can use it:

// 1. Declare variables
pthread_rwlockattr_t attr;
pthread_rwlock_t     *rwlock_ptr; 
 
// Assume 'rwlock_ptr' points to a region of Shared Memory...
 
// 2. Initialize the attribute
pthread_rwlockattr_init(&attr);
 
// 3. SET THE SHARING ATTRIBUTE
// PTHREAD_PROCESS_SHARED allows any process with access to the memory 
// to use this lock. (Default is PTHREAD_PROCESS_PRIVATE)
pthread_rwlockattr_setpshared(&attr, PTHREAD_PROCESS_SHARED);
 
// 4. Initialize the lock using these attributes
pthread_rwlock_init(rwlock_ptr, &attr);
 
// 5. Clean up the attribute (no longer needed after init)
pthread_rwlockattr_destroy(&attr);
4. Important Note on cleanup

Just like mutexes, if a process crashes while holding a read-write lock in shared memory, the system does not automatically release it. The lock stays held, likely causing other processes to hang (deadlock). Robustness in IPC is a major topic, and often System V Semaphores (Chapter 11) are preferred if automatic cleanup is strictly required.

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